Known washing machines comprise a casing containing a washing tub in which there is a rotatable laundry drum in which the laundry to be washed can be loaded. A hydraulic circuit is provided for taking washing liquid, i.e. water and water mixed with a cleaning product, inside the washing tub and the laundry drum, so as to wash or rinse the laundry, and for draining, e.g. by using a drain pump, this liquid outside the machine. In other known washing machines drain pump is not provided and the washing liquid is drained from the tub by gravity.
During a washing cycle in a washing machine one or more dewatering phases, balancing phases, spinning phases and rinsing phases are performed. When the washing liquid is moved, for example by rotating the laundry drum, in particular when such liquid contains a detergent, a certain amount of foam is typically generated. If too much foam is generated in the washing tub or in the draining region thereof, then some phases of the washing cycle cannot be faultlessly performed.
In particular an high amount of foam can be very negative during the spinning and draining phase of the washing cycle (in which the laundry drum is rotated at high speed and the liquid present in the washing tub is drained from the machine), since the foam compromises a correct functioning of the drain pump (if provided) and it obstructs the draining of the liquid from the machine.
Some known methods for operating a washing machine are known, in which the amount of foam in the washing tub is monitored, in particular during the spinning phase, in order to start a foam reducing procedure if this amount is too high. In these known methods, the monitoring of the foam amount is performed by detecting the liquid level in the washing tub or in the draining region.
A problem which arises in the monitoring of the foam amount is that this monitoring can't be effectively performed if a closed liquid ring (called also water ring) is formed at circumferential walls of the laundry drum; when the drum is rotating at a certain rotation speed there is a threshold level of the liquid level in the washing tub at which the liquid starts rotating along the circumferential walls of the drum, in such a way to create a closed liquid ring. This liquid ring can't be easily drained by the drain circuit, since it tends to keep rotating along the circumferential walls of the drum. In this condition the liquid level in the tub or in the draining region can't be accurately measured, and since the foam monitoring is typically related to measuring the liquid level, also the foam amount can't be correctly measured. If a closed liquid ring is formed, the monitoring of the foam amount, and therefore any related foam reducing procedure, can't be correctly performed. Since the threshold level is smaller for higher rotation speeds, this problem is particularly present during the spinning phases of the washing cycle, when the rotation speed is higher.
In particular during the dewatering phase and/or the spinning phase of the washing cycle, the formation of a closed liquid ring at circumferential walls of the laundry drum should be therefore avoided, since it affects negatively the monitoring of the liquid level, and also the monitoring of the foam amount.